This invention relates to a method for growing semiconductor heterojunction devices and, more particularly, to a method for growing homogeneous materials in such devices.
The liquid phase epitaxial (LPE) method of crystal growth [H. Nelson, RCA Rev. 24, 603 (1963)] has been widely used to grow various homo- and heterostructure devices. Theoretical descriptions of the LPE crystal growth process are typically based on the assumption of thermodynamic equilibrium at the liquidus-solidus interface, and have been used to predict various first-order crystal growth properties (e.g., growth rate) with fairly good agreement with experimental data. These descriptions do not account, however, for any processes at the interface that may dominate the initial nonequilibrium phase of the growth process. Any anomalies during the first stage of LPE crystal growth have been hard to detect and have been comparatively unimportant, since most applications of LPE, until recently, have involved relatively thick layers (.gtoreq.0.1.mu.m). LPE growth initiation problems have become potentially more severe, however, with the decreasing dimensions characterizing contemporary devices and with the increasing variety of complicated alloys being grown, notably In.sub.1-x Ga.sub.x P.sub.1-z As.sub.z. Some difficulties related to initial growth kinetics have recently been reported, such as hetero-interface nonideality [M. Feng, L. W. Cook, M. M. Tashima, G. E. Stillman, and R. J. Blattner, Appl. Phys. Lett. 34, 697 (1979)] and compositional grading [R. J. Nelson, Appl. Phys. Lett. 35, 643 (1979)], but since conventional crystal growth techniques prohibit the study of processes occurring within the first few milliseconds of growth, such reports are more of a second-hand nature.
Four variations of the basic LPE process have been developed. In the supercooling technique, the substrate and the growth solution are cooled at a uniform rate to a temperature below the liquidus temperature (T.sub.l) of the solution (but not enough to cause spontaneous precipitation of solute in the supercooled solution), then brought into contact with the substrate and cooled without interruption at the same rate until growth is terminated by wiping the solution off the substrate. In the step-cooling technique, the substrate and solution are cooled to a fixed temperature T.sub.l, (again not enough for spontaneous precipitation), then brought into contact with the substrate and kept fixed in temperature until growth is terminated. The equilibrium-cooling technique employs the same cooling procedure as the supercooling technique, but the substrate and the solution are brought into contact as close to T.sub.l as possible, rather than below it. The two-phase-solution technique employs the same cooling procedure as the supercooling and equilibrium-cooling methods, but in this case the temperature is lowered far enough below T.sub.l for precipitation to occur before the substrate and solution are brought into contact. This technique May be regarded as an approximation to the equilibrium-cooling method, but it is complicated by the departure from equilibrium at the beginning of the growth and also by the deposition of the solute on the precipitates as well as on the substrate.
The LPE growth of binary, ternary, and quaternary III-V semiconductor alloys by either the step-cooling, equilibrium-cooling, or supercooling technique has been shown to be mainly controlled by diffusion of the solute in the column-III-element-rich liquidus solution toward the melt-substrate interface (i.e., diffusion-limited growth). The principal assumption made is that a state near thermodynamic equilibrium exists at the interface, implying that surface attachment kinetics are characterized by times much smaller than the growth period (typically no shorter than a few tenths of a minute). For relatively long growth times (t.gtoreq.ls), the thickness of the epitaxial layers, as expected, is consistent with diffusion-limited theory. However, it has been found by us that for t&lt;200ms, the growth rate deviates significantly from diffusion-limited theory (but remains well-behaved). The usual conditions near the melt-substrate interface no longer apply, and other solute-depletion processes largely determine the growth rate. Nevertheless, Auger depth profiles of InGaPAs layers grown for t&lt;200ms indicate no compositional grading in these epilayers. On the other hand, a comparison of the photoluminescence spectra of an InGaPAs layer grown for t&lt;200ms (.about.18ms) and a quaternary layer grown at the same temperature from a melt with the same liquidus composition for t&gt;200ms (.about.ls) clearly shows the recombination radiation energy of the two layers to be .about.15 meV different.
Such experimental data present a strong case for the existence of two distinct LPE growth mechanisms during growth by the step-cooling technique.
In summary form, the following appears to be the explanation of what is happening during the growth of an active heterojunction layer. Assuming that a four-component melt is being utilized to create the active region (i.e., In.sub.1-x Ga.sub.x P.sub.1-z As.sub.z) and is being grown via LPE on an InP structure, the major component of the melt is In, with Ga, P, and As being at much lower concentrations. At the initial time of contact between the melt and substrate, all of the constituent components of the melt are in proportion and orderly growth of the active layer on the substrate commences. However, after a very short time (i.e., &gt;200ms), the three minor components of the melt become depleted at the growth interface and their replenishment is limited by the diffusion times of those elements from more distant portions of the melt. (The In is not depleted because of its overwhelming quantity.) Thus until replenishment of the elements stabilizes into a steady-state (constant-slope) condition, there is a substantial variation of composition of the active layer with a resultant shift of the energy gap to a higher magnitude.
In devices employing such heterojunctions (i.e., lasers), the aforestated composition change dictates higher threshold currents and larger than desired power dissipations.